The present invention relates to a reactor container or reactor containment vessel provided with a dry well cooling system for use in a nuclear power plant.
A reactor container of a boiling water reactor (hereinafter referred to as BWR) is generally divided into a dry well and a wet well. A reactor pressure vessel (hereinafter referred to as RPV) and a reactor primary system are housed in the dry well, and a suppression pool filled with a suppression pool water is also located in the wet well. In accordance with the nuclear policy in various countries such as Japan, design of a nuclear container is obliged to meet with strict requirements in an anticipation of a loss of coolant accident caused by breakage of reactor primary pipings that is one design reference event.
The vapor discharged in the event of breakage of the reactor primary pipings is introduced from the dry well to the suppression pool through a vent pipe for condensation of the vapor so that a pressure increase in the reactor container is suppressed. When the water temperature of the suppression pool is raised due to the condensation of the vapor, a residual heat removal system (hereinafter referred to as an RHR system) is operated to remove the heat from the suppression pool and, hence, the water temperature of the suppression pool can always be kept low.
A dry well cooler is also provided for the purpose of removing the heat dissipated from the reactor container and the reactor primary system during a normal operation and for controlling the temperature in the dry well so as to be prevented from rising excessively and be held within a certain range.
A first example of the dry well cooling system comprises, as shown in FIG. 18, a heat exchanger 3 mounted in a dry well 2 inside a reactor container 1, a blower 4 and a duct, not shown, for circulating an atmosphere in the dry well 2 through the heat exchanger 3, and a normal cooling system 6 for introducing cooling water to the secondary side of the heat exchanger 3 and finally conveying the heat in the dry well 2 to the sea 5 outside the power plant. With such arrangements, the temperature in the dry well 2 is controlled during normal operation. A wet well is not shown but is arranged bellow the dry well in the reactor container.
The cooling water on the secondary side of the heat exchanger 3, disposed in the dry well 2, of the dry well cooling system is conveyed from an equipment cooling pump 7 to the heat exchanger 3 through an equipment cooling heat exchanger 8 and a circulating pipe 9. It is also arranged such that, if necessary, the cooling water can be conveyed to a load 10 for an RHR heat exchanger and a load 11 for equipment cooling as well. The secondary side of the equipment cooling heat exchanger 8 is cooled by conveying seawater 12 to the equipment cooling heat exchanger 8 through the operation of a seawater pump 13. The reason why the seawater is used for the BWR of the described type is that the BWR is usually located near the seaside.
Alternatively, the dry well cooling system comprises a plurality of heat exchangers mounted in a dry well, a plurality of blowers and ducts for circulating a gas in the dry well through the heat exchangers, and a cooling system for circulating cooling water on the secondary side of the heat exchanger and finally conveying the heat in the dry well to the sea outside the power plant, thereby controlling the temperature in the dry well during a normal operation of a nuclear reactor.
FIG. 19 is a block diagram showing a second example of the dry well cooling system for emergency. An atmosphere in a dry well 2 is taken out by a blower 4a disposed outside a reactor container 1, cooled by a heat exchanger 3a, and then returned to the dry well 2 again for removal of heat from the reactor container 1.
The cooling water on the secondary side of the heat exchanger 3a of the dry well cooling system for emergency is conveyed from a reactor equipment cooling pump 7 to the heat exchanger 3a through a reactor equipment cooling heat exchanger 8. It is also designed such that, if necessary, the cooling water of a reactor equipment cooling line 9 can also be conveyed to an RHR heat exchanger load 10 and a load 11 for equipment cooling as well. The secondary side of the reactor equipment cooling heat exchanger 8 is cooled by conveying the seawater 12 to the reactor equipment cooling heat exchanger 8 through the operation of a reactor equipment cooling seawater pump 13.
A nuclear power plant is designed in anticipation of an accident that heat cannot completely be removed from a reactor container because of an occurrence of an abnormal event and simultaneous failure of the RHR system, or an accident wherein a re-supply means of cooling water to the RPV fails to work at the same time as the occurrence of an abnormal event (hereinafter referred to as a severe accident), although the probability of such a severe accident is so very small as to be practically improbable. Even if a severe accident should occur, adequate countermeasures are taken to surely keep safety of the nuclear power plant.
If a severe accident leading to complete an outage of heat removal from a reactor container occurs and the outage continues for a long time, the interior of the nuclear container experiences a condition of high temperature and high pressure. In other words, vapor and incondensable gas at high temperature are built up in a dry well and a wet well to produce a high-temperature and high-pressure condition in the reactor container.
In anticipation of such a severe accident, it has been contemplated to install a nuclear container vent system for discharging vapor and incondensable gas at high temperature fills in the dry well and the wet well to open air from an exhaust tower through a suppression pool.
When a nuclear container vent system is installed and operated to discharge the atmosphere in the reactor container to open air, fission products (hereinafter referred to as FP) contained in the atmosphere in the reactor container are held in water of the suppression pool and almost no FP is contained in the gas discharged to the open air. However, the possibility that a small amount of FP may be discharged to the open air is not zero.
Meanwhile, it is now under consideration to use a cooler mounted in a dry well (hereinafter referred to as a dry well cooler) as means for removing heat from the reactor container. Heat removal using the dry well cooler has no possibility that the atmosphere in the reactor container is directly discharged to the open air. However, since the dry well cooler is intended to remove heat during the normal operation, sufficient heat removal from the reactor container cannot be expected in a condition under high temperature and high pressure in the event of a severe accident.
Further, since the dry well cooler generally shares a cooling system with the RHR system, there is a problem that if the RHR system should be disabled, the dry well cooler also does not work. Moreover, there is also provided a problem that reliability of the dry well cooler is low.